262 DOCS. 273, 274 NOVEMBER 1916 is determined by the masses present there and only by these masses. A specific inertia-generating envelope is not assumed; rather, all inertia-generating matter will consist of stars, as those in the portion of our universe accessible to our telescopes. This is compatible with the facts only when we imagine that the portion of the universe visible to us must be considered extremely small (with regard to mass) against the universe as a whole. This view played an important role for me psychologically, since it gave me the courage to continue to work at the problem when I absolutely could not find a way of obtaining covariant field equations.[6] Now that the covariant field equations have been found, no motive remains to place such great weight on the total relativity of inertia. I can then join you in putting it this way. I always have to describe a certain portion of the universe. In this portion the guv's (as well as the inertia) are determined by the masses present in the observed portion of space and by the guv's at the boundary. Which part of the inertia stems from the masses and which part from the boundary conditions depends on the choice of the boundary. In practice I must, and in theory I can make do with this, and I am not at all unhappy when you reject all questions that delve further. On the other hand, you must not scold me for being curious enough still to ask: Can I imagine a universe or the universe in such a way that inertia stems entirely from the masses and not at all from the boundary conditions? As long as I am clearly aware that this whim does not touch the core of the theory, it is innocent; by no means do I expect you to share this curiosity! Have a look at the printer’s proof I sent to Ehrenfest. The link between the relat. postulate and the energy conservation law emerges particularly clearly there.[7] Cordial greetings, yours, Einstein. 274. To Wilhelm Ostwald [Berlin, 6 November 1916] Highly esteemed Colleague,[1] I thank you cordially for the paper on color theory,[2] which I am reading with enchantment for the second time already. Science is indebted to you for a significant advance here. I want to present it to our colleagues in Rubens’s seminar;[3] they also will be delighted with it. With respectful regards, yours truly, A. Einstein.